Thank you for your answer. It turned out, that it actually worked and it was me, who could not perceive the difference immediately. I don't know, maybe it is my fault, or I have too high expectations. I've always had the impression (based on other people's opinions) that the Sound Canvas was the best there is. Probably Doom and Heretic are not the best games to fully utilize the capabilities of the Roland. With that said, I was a bit disappointed because the sound of the Vsc is much closer to the windows midi than I expected. In Doom a Yamaha db50xg daughter board sounds miles better. I have a Gravis Classic and a Yamaha daughter board but a Roland card is much too expensive. Maybe it's just that it is emulated and an actual card would sound better? We'll see. Thanks for your help anyways.

The Windows MIDI patches were licensed by MS from Roland and, in theory, sound fairly close to the virtual Sound Canvas, they are, after all, a version of it.
You can listen to the DOOM files played with a real SC here:
http://www.youtube.com/watch?v=316i0uV32WEand compare yourself.